As mobile devices have been increasingly developed and the demand for such mobile devices has increased, the demand for secondary batteries has sharply increased as an energy source for the mobile devices. Among such secondary batteries is a lithium secondary battery having high energy density and discharge voltage, into which much research has been carried out and which is now commercialized and widely used.
A secondary battery has attracted considerable attention as an energy source for power-driven devices, such as an electric bicycle (E-bike), an electric vehicle (EV), and a hybrid electric vehicle (HEV), as well as an energy source for mobile wireless electronic devices, such as a mobile phone, a digital camera, a personal digital assistant (PDA), and a laptop computer.
A small-sized battery pack, in which a battery cell is provided, is used for small-sized devices, such as a mobile phone and a digital camera. On the other hand, a middle or large-sized battery pack, in which a battery pack including two or more battery cells (hereinafter, occasionally referred to as a “multi-cell”) connected to each other in parallel and/or in series is provided, is used for middle or large-sized devices, such as a laptop computer and an electric vehicle.
A lithium secondary battery exhibits excellent electrical properties, such as large capacity and high discharge voltage; however, the lithium secondary battery has low safety. For example, when abnormal operations, such as overcharge, overdischarge, exposure to high temperature, and an electrical short circuit, of the lithium secondary battery occur, decomposition of active materials and an electrolyte, which are components of the battery, is caused with the result that heat and gas are generated and the high-temperature and high-pressure conditions caused by generation of the heat and the gas accelerate the above-mentioned decomposition. Eventually, a fire or explosion may occur.
For this reason, the lithium secondary battery is provided with a safety system, such as a protection circuit to interrupt electric current when the battery is overcharged or overdischarged or when overcurrent flows in the battery, a positive temperature coefficient (PTC) element whose resistance greatly increases so as to interrupt electric current when the temperature of the battery increases, and a safety vent to interrupt electric current or exhausting gas when pressure increases due to generation of the gas. In case of a small-sized cylindrical secondary battery, for example, the PTC element and the safety vent are usually disposed at the top of an electrode assembly (a generating element) having a cathode/separator/anode structure, which is mounted in a cylindrical container. In case of a small-sized prismatic or pouch-shaped secondary battery, on the other hand, the protection circuit module and the PTC element are usually mounted at the upper end of a prismatic container or a pouch-shaped case, in which the generating element is mounted in a sealed state.
The safety-related problem of the lithium secondary battery is even more serious for a middle or large-sized battery pack having a multi-cell structure. Since a plurality of battery cells is used in the multi-cell battery pack, abnormal operation of some of the battery cells may cause abnormal operation of the other battery cells with the result that a fire or explosion may occur, which may lead to a large-scale accident. For this reason, the middle or large-sized battery pack is provided with a safety system, such as a battery management system (BMS), to protect the battery cells from overcharge, overdischarge, and overcurrent.
Meanwhile, as the lithium secondary battery is continuously used, i.e. as the lithium secondary battery is repeatedly charged and discharged, the generating element and electrical connection members are gradually degraded. For example, degradation of the generating element leads to decomposition of electrode materials and the electrolyte, by which gas is generated. As a result, the battery cell (the container or the pouch-shaped case) gradually swells. In a normal state of the lithium secondary battery, an active controller, such as the BMS, detects overdischarge, overcharge, or overcurrent of the battery pack. In a case in which the detected overdischarge, overcharge, or overcurrent of the battery pack is excessive, the active controller interrupts electrical connection in the battery pack to lower the risk of the battery pack.
In a case in which the active controller is used as described above, however, it is necessary to supply electric current to the BMS. If no electric current is supplied to the BMS, the BMS may not protect the battery pack.
Therefore, there is a high necessity for technology that is capable of fundamentally securing safety of the battery pack while solving the above problems.